Field of the Invention
The present invention relates to a composite material comprising a magnesium alloy-based metal matrix and a carbon reinforcement formed from graphite or carbon filaments or fibers.
The incorporation of particles, fibers or mineral trichites into metal matrixes constituted by metals having a low density such as alloys based on aluminum or magnesium makes it possible to obtain composite materials having specific mechanical properties superior to those of conventional alloys, particularly with regards to the breaking strength, rigidity and the maintaining of these properties in fatigue and thermal cycling. Among the different reinforcements which can be incorporated into such metal matrixes,graphite or carbon fibers have a particular interest due to their excellent intrinsic characteristics, namely a very high breaking strength (up to 4400 MPa), very high rigidity (up to 800 GPa), a slightly negative thermal expansion coefficient and low density (1.76 to 2.18).
Thus, composite materials incorporating such carbon reinforcements have always been used in various industrial fields such as motor vehicles (pistons, jackets, rods), as well as the aeronautical and aerospace fields (structural elements for shuttles, supports for reflectors, etc.) and it is very probable that there will be significant advances with respect thereto over the next few years.
The known processes for producing such composite materials use solid phase methods or liquid phase methods. In the case of solid phase methods, the material can e.g. be produced by the extrusion of a mixture of the constituents or by heat compression of layers of fibers forming the reinforcement and which have previously been metallized.
The production procedures using liquid phase methods can consist of infiltrating the metal or melted alloy constituting the matrix, under a varying pressure, into woven or non-woven preforms, or mixing constituents in the pasty phase, optionally in the presence of an easily eliminatable binder. The methods involving the pressurized infiltration of the metal or melted alloy into unidirectional layers of long, stretched fibers or into woven preforms lead at present to materials having the highest mechanical characteristics.
However, when the infiltration method is used with light metals such as aluminum or magnesium, certain difficulties are encountered in obtaining a satisfactory connection or bond between the fibers of the reinforcement and the metal matrix.
Thus, in the case of aluminum-based matrixes, the infiltration of the aluminium or aluminium alloy into the layers or preforms constituted by carbon fibers causes certain problems, because carbon is not chemically stable in the presence of pure or alloyed aluminium. Thus, as soon as the temperature exceeds approximately 450.degree. C., a reaction occurs between the fibers and the matrix leading to the formation of aluminium carbide Al.sub.4 C.sub.3, which leads to a damage by puncturing of the surface of the fibers and to a rapid deterioration of their intrinsic mechanical characteristics.
Moreover, despite the chemical interaction leading to the formation of aluminium carbide,the wetting of the fibers by aluminium is very poor, which make it necessary to carry out infiltration under a high pressure, whilst the adhesion between the aluminium and the fibers in the composite remains low.
To overcome these difficulties various surface treatments have been developed for said carbon or graphite fibers, said treatments e.g. consisting of coating the surface of the fibers with an appropriate compound such as zirconium or titanium carbide, as described in U.S. Pat. No. 4,600,661 with reference to Japanese document 49-18891 and Nieh and Vidoz in J. of Am. Ceram. Soc., vol. 65, No. 5, May 1982 pp 227-230.
In U.S. Pat. No. 4,600,661 reaction between the fibers and the aluminium matrix is prevented by adding titanium or zirconium to the melted aluminium during infiltration, which leads to the formation on said fibers of a Ti or Zr carbide layer in place of aluminium carbide. However, this procedure encounters problems in controlling or monitoring the formation of said layer in such a way as to maintain the mechanical properties of the fibers.
In the article by Nieh et al, a Ti or Zr carbide layer is formed on the fibers before including them in the aluminium matrix, which makes it possible to monitor the formation of the layer and leads to a double layer of TiC and Ti.sub.4 Sn.sub.2 C.sub.2 favoring the wetting of the fibers by aluminium. However, the need to carry out such a treatment makes the production process more complicated and slows down the development of such a composite material.
In addition, development has taken place of composite materials having a carbon reinforcement using a magnesium-based metal matrix, because magnesium has two important advantages compared with aluminium. On the one hand, magnesium has a density well below that of aluminium (1.74 instead of 2.7), which can make it possible to obtain even higher specific mechanical characteristics. On the other, unlike in the case of aluminium, magnesium has an excellent chemical inertia relative to carbon, provided that the latter is sufficiently graphitized, which is the case with most commercially available reinforcing fibers. Therefore there is no significant deterioration of the intrinsic characteristics of a reinforcement constituted by such fibers due to a chemical reaction with the metal of the matrix, during the hot production of composite materials using a matrix based on magnesium and a carbon reinforcement.
Nevertheless, although the carbon fibers are not deteriorated by the chemical reaction with the magnesium, two major problems remain for the production of such composite materials. The first is to allow a uniform penetration of the liquid magnesium in the reinforcing fibers and the second is to obtain an adequate adhesion at the metal/fiber interface.
In order to solve these two problems, various treatments of the reinforcing fibers have been proposed and in particular treatments consisting of coating the surface of the fibers with a metal, e.g. nickel, or insertion compounds wettable by the liquid matrix, as described in Chemical Abstracts, vol. 86 (2), No. 7835k and FR-A-2 259 916.
Titanium-based coatings have also been produced on graphite fibers intended to be infiltrated by a liquid magnesium alloy, as described in Chemical Abstracts, vol. 106 (20), No. 161024h and vol. 106 (8), No. 54243g. However, certain difficulties are encountered in obtaining with such treatments a uniform coating of each fiber of the carbon reinforcement. Moreover, the need to carry out a preliminary treatment on the fibers, prior to their infiltration by the metal of the matrix, makes the composite metal production process more complicated.
A process for producing a composite material combining a magnesium-based matrix and a carbon reinforcement is known, in which use is made for the matrix of a magnesium alloy containing 2 to 8% by weigh zinc, less than 2% by weight zirconium and less than 1% by weight aluminium, in order to avoid a deterioration of the carbon reinforcement by the alloy of the matrix and therefore increase the mechanical strength of the resulting composite material, as described in U.S. Pat. No. 4,600,661. In this process, the addition of zinc to the magnesium alloy lowers the melting point of the latter and improves its fluidity in the molten state, which makes it possible to bring about the infiltration under pressure of the carbon reinforcement by the magnesium alloy under better conditions. In addition, the limitation of the zirconium and aluminium contents of the alloy makes it possible to avoid what are considered to be prejudicial reactions between the Al, Zr and carbon of the carbon reinforcement.